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IanL

Nice Illustration of Radial Velocity vs. Doppler Shift

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Checking my meteor data for a presentation, and found this nice example of radial velocity vs. Doppler shift using two ISS passes about a day apart. You can see the ground track on the map is at a different angle relative to the GRAVES transmitter (centre of circle) and receiver (here in Essex) and so the frequency of the radar trace changes correspondingly as the ISS motion across our line of sight is slower on the 12th leading to a slower rate of frequency change. (The dates/times between the trace and map are different due to time zones and also my PC clock being 8 minutes slow for some reason).1629509879_ISSRadialVelocity.thumb.gif.4a0454e53b68397c47b44b04695ff06a.gif

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That's an interesting one Ian.  I captured the same two passes, but didn't notice the difference in Doppler until I saw your post.

I will need to think about this a bit: the gradients (i.e. the rate of change in Doppler shift), appear on face value to be different to yours.  However, I think this is to be expected.  Since my range to the ISS was greater, yet the absolute path taken by the ISS was the same, it traversed a smaller angle of my sky than yours in the same amount of time, so the radial velocity would have been lower and the gradient less steep.....I think.

965570437_ISS11th-12thJan.gif.6f8f495c600ba9cadc5db490403ab4dd.gif

Richard

 

 

Edited by BiggarDigger
Removed additional image

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That sounds right. When I have time I'm going to work through the maths.  Given we know the distance to the ISS from both transmitter and receiver and we know the absolute velocity of the ISS it should be possible to determine the rate of change of the frequency.

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Could I ask for a few more words regarding the data captured here and a brief explanation of the interpretation. I have some physics and I can appreciate the principle of the Doppler effect here.  An appropriate web link would suffice also.  Is that frequently range in the order of 1.4 x 10(8)  MHz or Hz. 

Thanks. 

 

 

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These are FFT waterfall plots of radar reflections off the International Space Station from the GRAVES satellite tracking radar located in France. Graves transmits on 143.05 MHz, and on my plots (first post) the vertical axis is 2 kHz wide centred on that frequency. Horizontal axis is 1 second intervals, most recent at the right.

Richard's plots are 2.2 kHz wide and five second intervals.

Doppler shift is easily understood for an object approaching directly towards you or moving directly away. Less obvious is the case where the object is moving at 90 degrees to that direct path, i.e. across your line of sight.

In such a case one might naïvely assume there is no Doppler shift, but that isn't the case. The only way an object could have no Doppler shift is if it follows a circular path with you at the centre. In that case it is neither approaching nor receding.

For any other path (straight or curved) and/or observer location, the object is either receding or approaching, except for the instant of closest approach. Thus it exhibits a Doppler shift that is a function of its absolute velocity (in some frame of reference, e.g. orbital velocity for the ISS) and location on its path relative to the observer, i.e. its "Radial Velocity".

For these forward scatter radar reflections one has to consider radial velocity relative to the transmitter, as the incoming radar signal will appear Doppler shifted as it arrives, plus the radial velocity relative to the receiver which creates a second shift from the receiver's perspective.

Read some of the posts in this forum, as there are links to maths, etc. which may be of interest.

Also, note that the Radar scans in Azimuth so the reflections are dashed lines as the radar scans on and off the ISS.

Edited by IanL

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On 03/02/2019 at 15:47, BiggarDigger said:

That's an interesting one Ian.  I captured the same two passes, but didn't notice the difference in Doppler until I saw your post.

I will need to think about this a bit: the gradients (i.e. the rate of change in Doppler shift), appear on face value to be different to yours.  However, I think this is to be expected.  Since my range to the ISS was greater, yet the absolute path taken by the ISS was the same, it traversed a smaller angle of my sky than yours in the same amount of time, so the radial velocity would have been lower and the gradient less steep.....I think.

965570437_ISS11th-12thJan.gif.6f8f495c600ba9cadc5db490403ab4dd.gif

Richard

 

 

Just realised that you are using 5 second time intervals and I am using 1 second so your waterfall plots may be differently scaled. Would have to rescale one set of images so that it has the same number of pixels per second (H) and pixels per Hz (V) to compare visually.

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3 hours ago, IanL said:

These are FFT waterfall plots of radar reflections off the International Space Station from the GRAVES satellite tracking radar located in France

Thanks for the excellent summary.  

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8 hours ago, IanL said:

Just realised that you are using 5 second time intervals and I am using 1 second so your waterfall plots may be differently scaled. Would have to rescale one set of images so that it has the same number of pixels per second (H) and pixels per Hz (V) to compare visually.

Ahh, I didn't catch that rather important observation Ian!

Taking your trace on the 11th, there is a 2Khz change in frequency in 17 approximately seconds.  Cropping mine to the same 2Khz vertical window (centred on the 0Hz Doppler shift point), I recorded it for 40 seconds.

In the image below, I've scaled my horizontal axis to the same as your's and crudely overlaid the two images.  My image is very wide at that scale, so I've aligned the two at the -1Khz point (the point at which it drops off the bottom of the waterfall plot).  That point is somewhat dependent on the accuracy of the RF tuning - hence the misalignment of the GRAVES azimuth switching pulses.  Also, there's some scaling errors left to right across the image, but even with those and the crude overlay, you can see a very large difference in radial velocity between your trace and mine.  Perhaps with more time (and an earlier evening) it may be possible to make a better job of the alignment.

 

image.png.da4329f6d85daf9d3f655cf4c9733cc9.png

Visually, I'd estimate that the ISS ran across about 90° of your sky compared to approximately 60° of mine at the same absolute velocity, meaning the magnitude of your radial velocity was much higher than mine.  I think!

Richard

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